Electrically powered vehicles are attractive in many application areas including civilian transport, military transport, long-life sensor platforms, undersea vehicles, airborne vehicles, and watercraft. In many cases, however, the operating time of these vehicles is short due to a drain on their storage systems by power intensive activities. As a result, their useful operating time is dictated by the ability to resupply them with electrical power. Electrical power can be supplied from either onboard power generation equipment or power transfer and storage of externally generated energy. On-board power generation is faced with many challenges, however. As a result, power transfer and storage systems are typically employed for most electrically powered vehicle systems.
In most cases, the capacity of the storage systems used to power these vehicles is limited; therefore, it is typically necessary to recharge these systems frequently. The time required to recharge an storage system can rival the operational time of the vehicle between charges. As a result, the use of electrically powered vehicles remains fairly limited. To further complicate matters, in many cases, the vehicle must be recharged without removing it from its environment, such as extended underwater sensor systems and Autonomous Undersea Vehicles (AUVs).
The transfer of externally generated electrical energy requires an ability to couple the external power source to an storage system on board the vehicle through a power coupling. Although underwater power couplings have been in use for a variety of underwater applications (e.g., oil industry, ships, submarines, towed arrays, etc.) for close to one hundred years, there are drawbacks to all known approaches.
Traditional contact-type power couplings (e.g., plug-and-socket connectors) suffer from a combination of complex connector geometries. Further, they are highly susceptible to corrosion when exposed to seawater. Although this type of coupling has been relied upon for many years, there is need for improvement in both the reliability of the power connection and its ease of use.
A variety of non-contact-type power couplings are known in the art, such as capacitive couplings, inductive couplings, radio frequency (RF) transformers, and resonant RF power couplings. Capacitive couplings generally suffer from relatively high impedance, which limits their power transfer efficiency. In addition, capacitive couplings require frequencies in excess of 100 megahertz to over a gigahertz to achieve kilowatt levels of power transfer.
Inductive (transformer) power couplings are more amenable to high power levels, but are based on very heavy core materials and require large amount of copper. As a result, inductive power couplings tend to be unwieldy and expensive to implement.
Radio Frequency (RF) transformers are much lighter than inductive couplings, but their transfer efficiency in a seawater environment is severely degraded by the conductivity of seawater itself.
Resonant RF power transfer has proven attractive for the transfer of electrical power over long distances. For example, resonant RF power transfer has been demonstrated to produce as high as 30 percent efficiency at multi-meter ranges in air. Unfortunately, the efficiency of resonant RF power transfer in seawater is also severely degraded by the conductivity of seawater. Further, the efficiency of prior-art resonant RF power coupling systems is reduced due to their reliance on open resonators, which radiate RF energy in many directions.